Ferrocene–Dithiolene Hybrids: Control of Strong Donor–Acceptor Electronic Communication to Reverse the Charge Transfer Direction

We prepared a novel class of ferrocene–dithiolene hybrid molecules, FcS4dt­(Me)2 and FcS4dt­[Pt­(tBu2bpy)] (where FcS4dt indicates 2-(1,3-dithia[3]­ferrocenophane-2-ylidene)-1,3-dithiole-4,5-dithiolate and tBu2bpy indicates 4,4′-di-tert-butyl-2,2′-bipyridine), in which the ferrocene moiety was bound to the planar conjugated dithiolene skeleton via two sulfur atoms such that the cyclopentadienyl rings were perpendicular to the dithiolene backbone. The physical properties and electronic structures of the complexes and their oxidized species [FcS4dt­(Me)2]•+ and [FcS4dt­[Pt­(tBu2bpy)]]•+ were investigated by means of single-crystal X-ray diffraction (XRD) analysis, cyclic voltammetry, electron paramagnetic resonance (EPR), and UV–vis near infrared (UV–vis–NIR) spectroscopy. The electron density distributions of the highest occupied molecular orbitals (HOMOs) of FcS4dt­(Me)2 and FcS4dt­[Pt­(tBu2bpy)] differed remarkably in that the HOMO of the former was ferrocene-based whereas that of the latter was dithiolene-based. The differences in the HOMO distributions originated from the energy level of the dithiolene-based π-orbital in each of the complexes, which was controlled by changing R in FcS4dt­(R)2 (R = Me for FcS4dt­(Me)2; 2R = Pt­(tBu2bpy) for FcS4dt­[Pt­(tBu2bpy)]). We succeeded in analyzing the crystal structure of [FcS4dt­[Pt­(tBu2bpy)]]­(F4TCNQ)·C6H14·CH2Cl2 (where F4TCNQ indicates 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), which provided a rare example of the crystal structure of a [Pt­(diimine)­(dithiolate)]•+ ion-based complex. A comparison of the bond lengths in FcS4dt­[Pt­(tBu2bpy)] and [FcS4dt­[Pt­(tBu2bpy)]]•+ suggested that the latter complex displayed a conjugated dithiolene-based π-radical character. These considerations agreed well with the electronic structures calculated using density functional theory (DFT) and time-dependent­(TD)-DFT methods. Significant electronic communication between the ferrocene and dithiolene moieties was detected for both [FcS4dt­(Me)2]•+ and [FcS4dt­[Pt­(tBu2bpy)]]•+ in the appearance of an intramolecular charge transfer band, which was hardly observed for previously reported ferrocene–dithiolene hybrid molecules. The charge transfer direction was reversed between the two cations. The electron coupling parameter HAB and the potential energy curves of the oxidized complexes were estimated based on the classical two-state Marcus–Hush theory. These results suggest that FcS4dt-based metalladithiolenes can exhibit controllable electronic structures expressed as double-minimum potential energy curves.本研究合成了一类新型二茂铁-二硫烯杂化分子：FcS4dt(Me)₂与FcS4dt[Pt(tBu₂bpy)]，其中FcS4dt指2-(1,3-二硫杂[3]二茂铁环蕃-2-亚基)-1,3-二硫杂环戊烯-4,5-二硫醇根，tBu₂bpy指4,4'-二叔丁基-2,2'-联吡啶。该类分子中二茂铁单元通过两个硫原子与平面共轭二硫烯骨架相连，使得环戊二烯基环与二硫烯主链呈垂直取向。本研究通过单晶X射线衍射（XRD）分析、循环伏安法、电子顺磁共振（EPR）及紫外-可见-近红外（UV–vis–NIR）光谱，表征了该系列配合物及其氧化产物[FcS4dt(Me)₂]•+与[FcS4dt[Pt(tBu₂bpy)]]•+的物理性质与电子结构。FcS4dt(Me)₂与FcS4dt[Pt(tBu₂bpy)]的最高占据分子轨道（HOMO）电子密度分布存在显著差异：前者的HOMO以二茂铁单元为电子密度中心，后者则以二硫烯骨架为电子密度中心。HOMO分布的差异源于各配合物中二硫烯基π轨道的能级，而该能级可通过调控FcS4dt(R)₂中的取代基R实现：FcS4dt(Me)₂的R为甲基，FcS4dt[Pt(tBu₂bpy)]的2R为Pt(tBu₂bpy)单元。我们成功解析了[FcS4dt[Pt(tBu₂bpy)]](F4TCNQ)·C₆H₁₄·CH₂Cl₂的晶体结构，其中F4TCNQ即2,3,5,6-四氟-7,7,8,8-四氰基对苯二醌二甲烷，该结构为基于[Pt(二亚胺)(二硫醇盐)]•+离子的配合物晶体结构的罕见范例。对比FcS4dt[Pt(tBu₂bpy)]与其氧化产物的键长可知，后者具有共轭二硫烯基π自由基特征。上述结论与通过密度泛函理论（DFT）及含时密度泛函理论（TD-DFT）计算得到的电子结构结果高度吻合。对于[FcS4dt(Me)₂]•+与[FcS4dt[Pt(tBu₂bpy)]]•+两种阳离子，均观测到分子内电荷转移带，表明二茂铁与二硫烯单元间存在显著的电子耦合作用，而这类电荷转移带在此前报道的二茂铁-二硫烯杂化分子中几乎未被观测到。且两种阳离子的分子内电荷转移方向恰好相反。基于经典双态马库斯-赫希（Marcus–Hush）理论，我们估算了氧化态配合物的电子耦合参数HAB与势能曲线。上述结果表明，基于FcS4dt的金属二硫烯配合物可展现出以双极小势能曲线表征的可控电子结构。